Various techniques are known for monitoring transmissions from signal sources such as a television station, a radio station, satellite and cable television providers (referred to collectively hereinafter as “broadcast source”). The signal of interest might be a program being transmitted by cable or satellite, or it might be a recorded program being played back from a CD, DVD or VCR. The program may be a “show” providing musical or dramatic entertainment, or it might be a commercial. The monitoring is carried out to provide information that, for example, reveals the size of the audience tuned to a given broadcast source at a given time of day, determines the total number of people who have heard or seen a program, provides independent validation that a commercial has been broadcast, and so on. Such information is useful for broadcasters, advertisers, etc. As used hereinafter, the term “program signal” is intended to include all such signals, be they, for example, a real time broadcast or one that has been recorded, to be suitably reproduced to be electronically performed for listening or viewing by an audience of a show or a commercial about which such information is being collected.
One approach that has been adopted to perform such monitoring is to combine the audio portion of a program signal with a code signal at the broadcasting end. The combined signal is made available, such as by on-air broadcast, to an intended audience. A receiver at the audience end detects the combined signal, uses the program signal to perform the program, and uses the code signal for audience monitoring. This is disclosed, for example, in U.S. Pat. Nos. 4,718,106, 5,457,807 and 5,630,203.
An audio signal for voice audio from a radio program is contained within a relatively narrow frequency spectrum. AM talk radio has a particularly narrow spectrum. The voice audio signal is vulnerable to extraneous and disturbing sounds if it is encoded with a monitoring code. Normal gaps that occur during speech are particularly vulnerable to encoding anomalies resulting from encoding the voice audio signal with the monitoring code. For example, by encoding the voice audio signal with the monitoring code, an instance of the monitoring code that occurs during a normal gap in speech in the voice audio signal may cause a listening audience to experience an extraneous and disturbing sound.
Many radio commercials contain only voice audio, and these voice audio commercials are often broadcast in real-time. Furthermore, voice audio commercials are constantly changed, and there is often no time to encode the voice audio with the monitoring code.